Ceramic composite articles with shape replicated surfaces

ABSTRACT

A method for producing a self-supporting ceramic composite body having a negative pattern which inversely replicates the positive pattern of a parent metal precursor having thereon a positive pattern section which is emplaced in conforming engagement with a bed of conformable filler. The parent metal precursor, which also has a non-replicating section, is melted and reacted with an oxidant to form a polycrystalline oxidation reaction product which grows primarily only from the positive pattern section of the parent metal precursor and through the filler. The molten parent metal is drawn through the growing polycrystalline material and oxidized at the interface between the oxidant and previously formed oxidation reaction product. The reaction is continued for sufficient time to at least partially embed the filler within the oxidation reaction product and form the ceramic composite body containing a negative pattern which inversely replicates the positive pattern of the parent metal precursor.

This is a continuation of copending application Ser. No. 07/308,420filed on Feb. 08, 1989, abandoned, which is a Rule 60 division of U.S.Pat. 4,859,640, which issued on Aug. 22, 1989, from U.S. Ser. No.06/896,157, filed on Aug. 13, 1986.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention broadly relates to ceramic composite bodies havinga shape replicated portion thereof and to methods of making the same. Inparticular, the invention relates to ceramic composite bodies comprisinga polycrystalline matrix embedding a filler and having a negativepattern shaped by inverse replication of a positive pattern of a parentmetal precursor, and to methods of making the composites by infiltratinga bed of filler with the oxidation reaction product of the parent metalprecursor, the positive pattern of which is inversely replicated to formthe negative pattern of the ceramic composite.

2. Description of Commonly Owned Patent Applications

The subject matter of this application is related to that of copendingand Commonly Owned U.S. patent applications Ser. No. 819,397, filed Jan.17, 1986, now U.S. Pat. No. 4,851,375 which is a continuation-in-part ofSer. No. 697,878, filed Feb. 4, 1985, both in the name of Marc S.Newkirk et al and entitled "Composite Ceramic Articles and Methods ofMaking Same." U.S. Pat. No. 4,851,375 discloses a novel method forproducing a self-supporting ceramic composite by growing an oxidationreaction product from a parent metal into a permeable mass of filler.The resulting composite, however, has no defined or predeterminedconfiguration.

The method of growing a ceramic product by an oxidation reaction isdisclosed generically in copending Commonly Owned U.S. patentapplications Ser. No. 818,943, filed Jan. 15, 1986, now U.S. Pat. No.4,713,360 as a continuation-in-part of Ser. No. 776,964, filed Sep. 17,1985, which is a continuation-in-part of Ser. No. 705,787, filed Feb.26, 1985, which is a continuation-in-part of Ser. No. 591,392, filedMar. 16, 1984, all in the name of Marc S. Newkirk et al and entitled"Novel Ceramic Materials and Methods of Making The Same." The employmentof an unusual oxidation phenomenon as described in the aforesaidPatents, which may be enhanced by the use of an alloyed dopant, affordsself-supporting ceramic bodies grown as the oxidation reaction productfrom a precursor parent metal and a method of making the same. Themethod was improved upon by the use of external dopants applied to thesurface of the precursor parent metal as disclosed in Commonly OwnedU.S. applications Ser. No. 220,935, filed Jun. 23, 1988, now U.S. Pat.No. 4,853,352, which is a continuation of Ser. No. 822,999, filed Jan.27, 1986, now U.S. Pat. No. 4,828,785, which is a continuation-in-partof Ser. No. 776,965, filed Sep. 17, 1985, which is acontinuation-in-part of Ser. No. 747,788, filed Jun. 25, 1985, which isa continuation-in-part of Ser. No. 632,636, filed Jul. 20, 1984, all inthe name of Marc S. Newkirk et al and entitled "Methods of MakingSelf-Supporting Ceramic Materials".

A method of forming ceramic bodies having one or more shaped cavitiestherein is disclosed in copending and Commonly Owned U.S. patentapplication Ser. No. 823,542, filed Jan. 27, 1986 in the name of Marc S.Newkirk et al and entitled "Inverse Shape Replication Method of MakingCeramic Composite Articles and Articles Obtained Thereby". The cavityformed in the ceramic body inversely replicates the shape of a positivepattern or mold of the parent metal which is embedded within andentirely surrounded by a conformable filler which is sufficientlyconformable to accommodate differential thermal expansion between thefiller and the parent metal plus the melting point volume change of themetal, and which self-bonds at an appropriate temperature to insure thatthe cavity formed by migration of molten parent metal into the filler(to form oxidation reaction product) does not collapse due to thepressure differential created across the developing cavity wall as aresult of the cavity-forming migration.

The entire disclosure of each of the foregoing Commonly Owned Patentsand patent applications is expressly incorporated herein by reference.

BACKGROUND AND PRIOR ART

In recent years, there has been increasing interest in the use ofceramics for structural applications historically served by metals. Theimpetus for this interest has been the superiority of ceramics withrespect to certain properties, such as corrosion resistance, hardness,modulus of elasticity, and refractory capabilities, when compared withmetals.

Current efforts at producing higher strength, more reliable, and tougherceramic articles are largely focused upon (1) the development ofimproved processing methods for monolithic ceramics and (2) thedevelopment of new material compositions, notably ceramic matrixcomposites. A composite structure is one which comprises a heterogeneousmaterial, body or article made of two or more different materials whichare intimately combined in order to attain desired properties of thecomposite. For example, two different materials may be intimatelycombined by embedding one in a matrix of the other. A ceramic matrixcomposite structure typically comprises a ceramic matrix which enclosesone or more diverse kinds of filler materials such as particulates,fibers, rods or the like.

The traditional methods of preparing ceramic articles involve thefollowing general steps: (1) preparation of ceramic material in powderform; (2) grinding or milling of powders to obtain very fine particles;(3) formation of the powders into a body having the desired shape (withallowance for shrinkage during subsequent processing), for example, byuniaxial pressing, isostatic pressing, injection molding, tape casting,slip casting or any of several other techniques; (4) densification ofthe body by heating it to an elevated temperature such that theindividual powder particles merge together to form a coherent structure;preferably, accomplished without the application of pressure (i.e., bypressureless sintering), although in some cases an additional drivingforce is required and can be provided through the application ofexternal pressure either uniaxially (i.e., hot pressing) orisostatically, i.e., hot isostatic pressing; and (5) finishing,frequently by diamond grinding, as required.

A considerable amount of current work is directed toward improved powderprocessing technologies. The emphasis in such developments has been intwo areas: (1) improved methods of producing ultrafine, uniform powdermaterials using sol-gel, plasma and laser techniques, and (2) improvedmethods of densification and compaction, including superior techniquesfor sintering, hot pressing and hot isostatic pressing. The object ofthese efforts is to produce dense, fine-grained, flaw-freemicrostructures, and, in fact, improvements in performance capabilitiesin ceramics have been attained in some areas. However, thesedevelopments tend to result in dramatic increases in the cost ofproducing ceramic structures. Thus, cost becomes a major restriction onthe commercial application of ceramics.

Another limitation in ceramic engineering which is aggravated by modernceramic processing is scaling versatility. Conventional processes aimedat densification (i.e., removal of voids between powder particles) areincompatible with large one-piece structural application possibilitiesfor ceramics. An increase in article size presents several problemsincluding, for example, increased process residence times, stringentrequirements for uniform process conditions over a large process volume,cracking of parts due to non-uniform densification or thermally inducedstresses, warping and sagging of parts during sintering, excessivecompaction forces and die dimensions if hot pressing is used, andexcessive pressure vessel costs due to internal volume and wallthickness requirements in the case of hot isostatic pressing.

When these traditional methods are applied to the preparation of ceramicmatrix composite materials, additional difficulties arise. Perhaps themost serious problems concern the densification step, number (4) above.The normally preferred method, pressureless sintering, can be difficultor impossible with particulate composites if the materials are nothighly compatible. More importantly, normal sintering is impossible inmost cases involving fiber composites even when the materials arecompatible, because the merging together of the matrix particles isinhibited by the fibers which tend to prevent the necessarydisplacements of the densifying powder particles. These difficultieshave been, in some cases, partially overcome by forcing thedensification process through the application of external pressure athigh temperature. However, such procedures can generate many problems,including breaking or damaging of the reinforcing fibers by the externalforces applied, limited capability to produce complex shapes (especiallyin the case of uniaxial hot pressing), and generally high costsresulting from low process productivity and the extensive finishingoperations sometimes required.

Additional difficulties can also arise in the blending of powders withwhiskers or fibers and in the body formation step, number (3) above,where it is important to maintain a uniform distribution of thecomposite second phase within the matrix. For example, in thepreparation of a whisker-reinforced ceramic composite, the powder andwhisker flow processes involved in the mixing procedure and in theformation of the body can result in non-uniformities and undesiredorientations of the reinforcing whiskers, with a consequent loss inperformance characteristics.

The Commonly Owned Patent Applications describe new processes whichresolve some of these problems of traditional ceramic technology asdescribed more fully therein, including the formation of cavities, whichmay be of complex shape, by inverse replication of a pre-shaped parentmetal precursor mold. The present invention combines these processeswith additional novel concepts to provide for the formation of ceramicbodies, including complex structures, to net or near net shape, by atechnique which does not require the utilization of self-bondingfillers. This invention also provides great flexibility in selecting thepattern or pattern to be replicated, including shapes having re-entrantformations, e.g., recesses or cavities, having mouths which are ofsmaller diameter or width than their interiors. In other words, themethod of the present invention is not limited to producing shapes whichcan be withdrawn from a die or mold. When making ceramic articles havingsuch re-entrant formations, prior art methods utilizing step (3) aboveoften are not feasible, because the internal pattern or mold cannot beremoved after the ceramic body is formed around it.

The present invention provides for fabrication of ceramic composites ofa predetermined shape by an unusual oxidation phenomenon which overcomesthe difficulties and limitations associated with known processes. Thismethod provides shaped ceramic bodies typically of high strength andfracture toughness by a mechanism which is more direct, more versatileand less expensive than conventional approaches.

The present invention also provides means for reliably producing ceramicbodies having shaped configurations of a size and thickness which aredifficult or impossible to duplicate with the presently availabletechnology.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method forproducing a self-supporting ceramic composite body having a negativepattern which inversely replicates a positive pattern of a parent metalprecursor. The ceramic composite body comprises a ceramic matrix havinga filler embedded therein, the matrix being obtained by oxidation of aparent metal to form a polycrystalline material which consistsessentially of the oxidation reaction product of said parent metal withan oxidant, e.g., with a vapor-phase oxidant, and, optionally, one ormore metallic constituents. The method comprises the following steps:The parent metal precursor, which has a positive pattern section forinverse replication and a non-replicating section, is emplaced inconforming engagement with a bed of conformable filler under growthcontrol conditions to promote growth of the oxidation reaction productfrom the positive pattern section, and to inhibit such growth from thenon-replicating section. The filler is permeable to the oxidant whenrequired (as in the case where the oxidant is a vapor-phase oxidant) topermit the oxidant to contact the molten parent metal as described belowand, in any case, is permeable to infiltration by the growth ofoxidation reaction product through the filler. The emplaced parent metalprecursor is heated to a temperature region above its melting point butbelow the melting point of the oxidation reaction product to form a bodyof molten parent metal, and in that temperature region, the moltenparent metal is reacted with the oxidant to form the oxidation reactionproduct. At least a portion of the oxidation reaction product ismaintained in that temperature region and in contact with and betweenthe body of molten metal and the oxidant, to progressively draw moltenmetal from the body of molten metal through the oxidation reactionproduct and into contact with the oxidant within the bed of filler foroxidation reaction therein. Concurrently therewith, the negative patternbegins to develop and eventually is formed in the bed of filler asoxidation reaction product continues to form at the interface betweenthe oxidant and previously formed oxidation reaction product. Thisreaction is continued in that temperature region for a time sufficientto at least partially infiltrate or embed the bed of filler within theoxidation reaction product by growth of the latter to form the compositebody having the aforesaid negative pattern. Finally, the resultingself-supporting ceramic composite body is separated from excess filler,and unreacted parent metal, if any.

Other aspects of the invention include one or more of the followingfeatures, alone or in combination: emplacing the parent metal precursorin engagement with the bed of conformable filler so that thenon-replicating section of the parent metal precursor is free fromcontact with the bed of filler; utilizing growth control conditionswhich comprise applying an external dopant to said positive patternsection; incorporating an oxidant into the conformable filler; using anon-oxidizing gas or vacuum process environment; and overlaying thenon-replicating section of the parent metal precursor with a barriermeans or growth-preventive means which inhibits growth of the oxidationreaction product therethrough. As used herein and in the appendedclaims, the term "inhibits growth" is broad enough to include themeaning "prevents growth". Further, as used herein and in the appendedclaims, reference to "applying an external dopant to said positivepattern section" or words of like import are to be understood to meanand include one or both of the following techniques: applying the dopantdirectly to selected surfaces of the parent metal precursor, andapplying the dopant on or to the conformable filler in an area thereoffacing, adjacent to or contiguous with the selected surfaces of theparent metal precursor.

In another aspect of the invention, the conformable filler is alsoself-bonding, at least when required to resist pressure differentialsformed across the oxidation reaction product by growth thereof.

In another aspect of the invention, there is provided a self-supportingceramic composite body having a negative pattern which inverselyreplicates the positive pattern of a parent metal mold or precursorhaving, in addition to a section comprising the aforesaid positivepattern, a non-replicating section. The ceramic composite body comprisesa polycrystalline matrix having incorporated therein a filler obtainedfrom a bed of conformable filler against which the parent metalprecursor is employed at an initial location with the positive patternthereof in conforming engagement with the filler and the non-replicatingsection thereof free from contact with the bed of filler. The positivepattern of the parent metal precursor is inversely replicated uponevacuation of the metal precursor from its initial location to form theinversely replicated negative pattern concurrently with oxidationreaction of molten parent metal precursor migrated from the initiallocation to form the polycrystalline matrix. The matrix consistsessentially of a polycrystalline oxidation reaction product of theparent metal precursor with an oxidant and, optionally, one or moremetallic constituents, or pores, or both, as described in more detailelsewhere herein.

The materials of this invention can be grown with substantially uniformproperties throughout their cross section to a thickness heretoforedifficult to achieve by conventional processes for producing shapedceramic structures. The process which yields these materials alsoobviates the high costs associated with conventional ceramic productionmethods, including fine, high purity, uniform powder preparation, greenbody forming, binder burnout, sintering, hot pressing and hot isostaticpressing. The products of the present invention are adaptable orfabricated for use as articles of commerce which, as used herein, isintended to include, without limitation, industrial, structural andtechnical ceramic bodies for such applications where electrical, wear,thermal, structural or other features or properties are important orbeneficial, and is not intended to include recycled or waste materialssuch as might be produced as unwanted by-products in the processing ofmolten metals.

As used in this specification and the appended claims, the terms beloware defined as follows:

"Ceramic" is not to be unduly construed as being limited to a ceramicbody in the classical sense, that is, in the sense that it consistsentirely of non-metallic and inorganic materials, but rather refers to abody which is predominantly ceramic with respect to either compositionor dominant properties, although the body may contain minor orsubstantial amounts of one or more metallic constituents derived fromthe parent metal, or reduced from the oxidant or a dopant, mosttypically within a range of from about 1-40% by volume, but may includestill more metal.

"Oxidation reaction product" generally means one or more metals in anyoxidized state wherein a metal has given up electrons to or sharedelectrons with another element, compound, or combination thereof.Accordingly, an "oxidation reaction product" under this definitionincludes the product of reaction of one or more metals with an oxidantsuch as those described in this application.

"Oxidant" means one or more suitable electron acceptors or electronsharers and may be a solid, a liquid or a gas (vapor) or somecombination of these (e.g., a solid and a gas) at the processconditions.

"Parent metal" refers to that metal, e.g., aluminum, which is theprecursor for the polycrystalline oxidation reaction product, andincludes that metal as a relatively pure metal, a commercially availablemetal with impurities and/or alloying constituents, or an alloy in whichthat metal precursor is the major constituent; and when a specifiedmetal is mentioned as the parent metal, e.g., aluminum, the metalidentified should be read with this definition in mind unless indicatedotherwise by the context.

"Negative pattern" of the ceramic composite body means the pattern(i.e., geometry) of the body which is inversely replicated from thepositive pattern (i.e., geometry) of the parent metal precursor.

"Positive pattern" of the parent metal precursor means the pattern(i.e., geometry) of the parent metal which is inversely replicated toform the negative pattern of the ceramic body. It is important to notethat the terms "negative" and "positive" are used in this context onlyin a sense relative one to the other to denote that the geometry of onepattern is congruent to that of the other. It is not intended in any wayto restrict the type of shapes which may comprise a "negative pattern"or a "positive pattern".

"Inversely replicated" means that the negative pattern of the ceramiccomposite body comprises surfaces which are congruent to the shape ofthe positive pattern section of the parent metal precursor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a parent metal precursor shaped toprovide on one side thereof a positive pattern and on the opposite sidethereof a non-replicating section;

FIG. 1A is a perspective view of the parent metal precursor of FIG. 1 ina position rotated 180° about its major longitudinal axis from itsposition in FIG. 1;

FIG. 2 is a schematic, cross-sectional view in elevation showing on aslightly reduced scale an assembly of the shaped parent metal precursorof FIGS. 1 and 1A emplaced within a refractory vessel at the interfacebetween a layer of conformable filler supporting a superposed layer ofparticulate inert material;

FIG. 3 is a perspective view of a ceramic composite body, after grindingthe rough surfaces thereof, in accordance with the invention and made byutilizing the assembly of FIG. 2 with the interface between the layersof filler and barrier material being at plane X--X;

FIG. 4 is a perspective section view of a ceramic composite body inaccordance with the invention, before grinding of the rough surfacesthereof, and made by utilizing the assembly of FIG. 2 with the interfacebetween the layers of filler and barrier material being at plane Y--Y;

FIG. 5 is a partial elevational view in partial section and on anenlarged scale of the parent metal precursor of FIGS. 1 and 1A, with alayer of external dopant applied to the positive pattern sectionthereof;

FIG. 6 is a schematic, cross-sectional view in elevation of an assemblyof a shaped parent metal precursor emplaced within a barrier meansenclosure and contained within a refractory vessel, with the positivepattern section of the parent metal precursor in conforming engagementwith a conformable filler;

FIG. 7 is a perspective view of a ceramic composite body in accordancewith the invention and made by utilizing the assembly of FIG. 6;

FIG. 8 is a perspective view of a parent metal precursor shaped so thatthe exterior surfaces thereof provide a positive pattern and the surfaceof the cylindrical bore extending therethrough provides anon-replicating section;

FIG. 8A is a perspective view of the parent metal precursor of FIG. 8 ina position rotated 180° about its major longitudinal axis from itsposition in FIG. 8;

FIG. 8B is a side view in elevation of the parent metal precursor ofFIGS. 8 and 8A with a cylindrical barrier means inserted within andprotruding from either end of the cylindrical bore of the precursor;

FIG. 9 is a schematic, cross-sectional view in elevation showing anassembly of the shaped parent metal precursor of FIG. 8B emplaced withina refractory vessel in an assembly including conformable filler andbarrier means;

FIG. 10 is a perspective view with parts broken away and sectioned of aparent metal precursor shaped similarly or identically to that of FIGS.1 and 1A and encased within a barrier means; and

FIG. 11 is a schematic, cross-sectional view in elevation showing anassembly of the shaped parent metal precursor and barrier means of FIG.10 emplaced within a refractory vessel in an assembly including aconformable filler and barrier means.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

In the practice of the present invention, the parent metal precursor isprovided in the form of a shaped article having one section comprised ofa positive pattern, the shape or geometry of which is to be inverselyreplicated as a negative pattern of a finished ceramic composite, and anon-replicating section. By following the practices of the presentinvention, negative patterns of complex shapes can be inverselyreplicated in the finished ceramic composite during formation or growthof the ceramic, rather than by shaping or machining a ceramic body. Theparent metal precursor may be suitably shaped by any appropriate means;for example, a piece of metal such as a bar, billet or ingot may besuitably machined, cast, molded, extruded or otherwise shaped to providethe parent metal precursor. The parent metal precursor may have grooves,bores, recesses, lands, bosses, flanges, studs, screw threads and thelike formed therein as well as having collars, bushings, discs, bars, orthe like assembled thereto to provide a positive pattern of the desiredconfiguration. The parent metal precursor may comprise one or moreunitary pieces of metal suitably shaped so that when emplaced with thepositive pattern section thereof in conforming engagement with aconformable bed of filler, (and the non-replicating section free of thebed of filler) the positive pattern defines a shaped segment of the bedof filler immediately adjacent to the mass of the parent metalprecursor. When the parent metal precursor is melted and the oxidationreaction product infiltrates the bed of filler, a shaped negativepattern develops in the resulting ceramic composite body. Thus, in oneaspect, the present invention provides the advantage of making thenegative pattern by machining or otherwise shaping a metal, rather thanby grinding or machining a ceramic, which is a much more difficult andcostly process.

In carrying out the method of the invention, the parent metal precursoris emplaced with its positive pattern section in conforming engagementwith a bed of conformable filler under growth control conditions whichwill promote growth of the oxidation reaction product primarily orexclusively from the positive pattern section and into the bed ofconformable filler, while inhibiting or preventing growth of oxidationreaction product from the non-replicating section. Growth controlconditions may be achieved or enhanced by establishing oxidationreaction kinetics of the parent metal which are more favorable adjacentto, or in the vicinity of, the positive pattern section than thoseadjacent the non-replicating section. The result is preferential growthor development of the oxidation reaction product within and into the bedof conformable filler from the positive pattern section and inhibitionor elimination of such growth from the non-replicating section. Forexample, a suitable external dopant may be applied onto or at thepositive pattern section which enhances growth from the portions of theparent metal precursor to which it is applied, as explained in detail inU.S. Pat. No. 4,853,352 described above. Such dopant may be appliedexternally to the surface of the positive pattern section of the parentmetal precursor and/or may be supplied in the conformable filler facingthe positive pattern section, preferably adjacent or contiguous to thesurface of the positive pattern section. Still further, a solid oxidantand/or liquid oxidant (explained below in detail) may be incorporatedinto the filler bed in the portion of zone adjacent the positive patternsection. Growth therefore will occur, or is facilitated, in thedirection of the oxidant.

Growth control of the polycrystalline oxidation reaction product can beachieved with a suitable barrier means or growth preventive means, suchas with the embodiments described in copending U.S. application Ser. No.861,024, filed May 8, 1986, now U.S. Pat. No. 4,923,832. Effectivebarriers include materials which are non-wettable by the transportedmolten parent metal under the process conditions, in that there isessentially no affinity of molten metal for the barrier and growththerefore is prevented. Barriers also may be used which tend to reactwith the transported molten parent metal to inhibit further growth. Inparticular, useful barriers include calcium sulfate, calcium silicate,portland cement, metal alloys, such as a stainless steel, and dense orfused ceramics, such as alumina, which may be used with aluminum as theparent metal. The barrier means may also include as a component thereofa suitable combustible or volatile material that is eliminated onheating, or a material which decomposes on heating, in order to renderthe barrier means permeable or to increase the porosity and permeabilityof the barrier means. The barrier means overlays or is superimposed ontothe non-replicating section of the parent metal, and preferably is of amaterial that will conform to the surface or shape of this sectionthereby minimizing or eliminating any undesired growth. A combination ofthe techniques may be employed, that is, a barrier means may be overlaidor superimposed on the non-replicating section of the parent metalprecursor and an external dopant applied to the positive pattern sectionand/or to the filler facing the positive pattern section. Thenon-replicating section of the parent metal precursor may be kept freeof the bed of filler even if it is not overlaid with a barrier materialor means, i.e., it may be left exposed to the atmosphere when conditionsare such that growth of oxidation reaction product in the atmosphere isinhibited or precluded except for those surfaces of the parent metalprecursor to which an external dopant, or solid or liquid oxidant, ismade available.

Although the invention is described below in detail with specificreference to aluminum as the preferred parent metal, other suitableparent metals which meet the criteria of the present invention include,but are not limited to, silicon, titanium, tin, zirconium and hafnium.

A solid, liquid or vapor-phase oxidant, or a combination of suchoxidants, may be employed, as noted above. For example, typical oxidantsinclude, without limitation, oxygen, nitrogen, a halogen, sulphur,phosphorus, arsenic, carbon, boron, selenium, tellurium, and compoundsand combinations thereof, for example, methane, ethane, propane,acetylene, ethylene, and propylene (as a source of carbon), SiO₂ (as asource of oxygen) and mixtures such as air, H₂ /H₂ O and CO/CO₂, thelatter two (i.e., H₂ /H₂ O and CO/CO₂) being useful in reducing theoxygen activity of the environment.

Although any suitable oxidants may be employed, specific embodiments ofthe invention are described below with reference to use of vapor-phaseoxidants. If a gas or vapor oxidant, i.e., a vapor-phase oxidant, isused the filler is permeable to the vapor-phase oxidant so that uponexposure of the bed of filler to the oxidant, the vapor-phase oxidantpermeates the bed of filler to contact the molten parent metal therein.The term "vapor-phase oxidant" means a vaporized or normally gaseousmaterial which provides an oxidizing atmosphere. For example, oxygen orgas mixtures containing oxygen (including air) are preferred vapor-phaseoxidants, as in the case where aluminum is the parent metal, with airusually being more preferred for obvious reasons of economy. When anoxidant is identified as containing or comprising a particular gas orvapor, this means an oxidant in which the identified gas or vapor is thesole, predominant or at least a significant oxidizer of the parent metalunder the conditions obtaining in the oxidizing environment utilized.For example, although the major constituent of air is nitrogen, theoxygen content of air is the sole or predominant oxidizer for the parentmetal because oxygen is a significantly stronger oxidant than nitrogen.Air therefore falls within the definition of an "oxygen-containing gas"oxidant but not within the definition of a "nitrogen-containing gas"oxidant. An example of a "nitrogen-containing gas" oxidant as usedherein and in the claims is "forming gas", which contains about 96volume percent nitrogen and about 4 volume percent hydrogen.

When a solid oxidant is employed, it may be dispersed through the entirebed of filler or, if used in conjunction with a vapor-phase oxidant,through a portion only of the bed adjacent the parent metal. The oxidantmay be used in particulate form admixed with the filler, and/or as acoating on the filler particles. Any suitable solid oxidant may beemployed including elements such as boron or carbon, or reduciblecompounds such as silicon dioxide (as a source of oxygen) or certainborides of lower thermodynamic stability than the boride reactionproduct of the parent metal. If a solid oxidant is used in combinationwith a vapor-phase oxidant, the oxidants are selected so that they willbe compatible for purposes of the invention.

If a liquid oxidant is employed, the entire bed of filler or a portionthereof adjacent the molten metal is coated, soaked as by immersion,dispersed or otherwise incorporated with the oxidant so as to impregnateall or part of the filler. Reference to a liquid oxidant means one whichis a liquid under the oxidation reaction conditions, and so a liquidoxidant may have a solid precursor, such as a salt, which is molten atthe oxidation reaction conditions. Alternatively, the liquid oxidant maybe a liquid precursor, e.g., a solution of a material which is used tocoat or impregnate part or all of the filler and which is melted ordecomposed at the oxidation reaction conditions to provide a suitableoxidant moiety. Examples of liquid oxidants as herein defined includelow melting glasses. If a liquid oxidant is used in combination with avapor-phase oxidant, the liquid oxidant should be used in such a mannerso as not to obscure access of the vapor-phase oxident to the moltenparent metal.

For certain conditions, it may be advantageous to employ a solid oxidantand/or a liquid oxidant in conjunction with the vapor-phase oxidant.Such a combination of additional oxidants may be particularly useful inenhancing oxidation of the parent metal to form the oxidation reactionproduct preferentially within the bed of filler, especially adjacent thepositive pattern, rather than beyond its surfaces or in thenon-replicating section. That is, the use of such additional oxidantswithin the bed of filler adjacent the positive pattern section maycreate an environment within that portion or zone of the bed which ismore favorable to oxidation kinetics of the parent metal than theenvironment outside that portion or zone of the bed. This enhancedenvironment is beneficial in promoting growth of the oxidation reactionproduct matrix within the bed to the boundary thereof and eliminating orminimizing overgrowth, i.e., growth outside the boundary of the bed offiller.

The conformable filler utilized in the practice of the invention may beone or more of a wide variety of materials suitable for the purpose. Asused herein and in the claims, the term "conformable" as applied to thefiller means that the filler is one which can be packed around, laid upagainst, or wound around a shaped parent metal precursor and willconform to the pattern or shape of the portions or sections of theprecursor against which it is emplaced in conforming engagement. Forexample, if the filler comprises particulate material such as finegrains of a refractory metal oxide, the positive pattern of the parentmetal precursor is emplaced in conforming engagement with the filler sothat the positive pattern defines a shape in the filler congruent to,i.e., the negative of, the positive pattern. However, it is notnecessary that the filler be in fine particulate form. For example, thefiller may comprise wire, fibers or whiskers, or such materials as metalwool. The filler also may comprise either a heterogeneous or homogeneouscombination of two or more such components or geometric configurations,e.g., a combination of small particulate grains and whiskers. It isnecessary only that the physical configuration of the filler be such asto permit the positive pattern of the parent metal precursor to beemplaced in conforming engagement against a mass of the filler with thefiller closely conforming to the surfaces of the positive pattern sothat the negative pattern ultimately formed in the composite body is thenegative of the positive pattern of the parent metal precursor. Thelatter thus initially forms a shaped segment of the bed of conformablefiller.

The conformable filler useful in the practice of the invention is onewhich, under the oxidation reaction conditions of the invention asdescribed below, is permeable to passage therethrough of the oxidantwhen the latter is a vapor-phase oxidant. In any case, the filler alsois permeable to the growth or development therethrough of oxidationreaction product. During the oxidation reaction, it appears that moltenparent metal migrates through the oxidation reaction product beingformed to sustain the reaction. This oxidation reaction product isgenerally impermeable to the surrounding atmosphere and therefore thefurnace atmosphere, e.g., air, cannot pass therethrough. As explained inthe aforesaid Commonly Owned Pat. No. 4,828,785 the impermeability ofthe growing oxidation reaction product to the furnace atmosphere resultsin a pressure differential problem when the oxidation reaction productencloses a cavity being formed by migration of molten parent metal. Thisproblem is overcome in the aforesaid Commonly Owned Patent by use of aself-bonding conformable filler which, as defined therein, is a fillerwhich, at a temperature above the melting point of the parent metal andclose to, but below, the oxidation reaction temperature, partiallysinters or otherwise bonds to itself and to the growing layer ofoxidation reaction product sufficiently to provide structural strengthfrom the outside of the growing cavity to retain the replicated geometryof the mold in the developing cavity at least until the growingoxidation reaction product structure attains sufficient thickness to beself-supporting against the pressure differential which develops acrossthe wall of growing oxidation reaction product defining the cavity beingformed. However, the self-bonding filler is not to sinter or self-bondat too low a temperature because, if it does, it could be cracked bythermal expansion and volume change upon melting of the parent metal asthe latter is heated to operating temperature. In other words, theself-bonding filler should retain its conformability to accommodate thedifference in volume changes between it and the parent metal while thelatter is being heated and melted and then self-bond to providemechanical strength to the developing cavity as the oxidation reactionprogresses. However, the technique of the present invention in manycases avoids the pressure-differential problem because the parent metalprecursor has a (non-replicating) section thereof from which oxidationreaction product is not grown, at least not to any significant degree,so there is not formed a cavity totally enclosed by growing oxidationreaction product. However, barrier means which are atmosphereimpermeable may be used and in some cases deployed so that they blockaccess of the furnace atmosphere to the forming cavity, resulting increation of a pressure differential across the walls of the growingoxidation reaction product. In such circumstances a self-bonding filleris employed to afford mechanical strength at least during the initialgrowth stage, as described above.

As used herein and in the claims to characterize conformable fillers,the term "self-bonding" means those fillers which, placed in conformingcontact with the positive pattern of the parent metal, retain sufficientconformability to accommodate melting point volume change of the parentmetal and differential thermal expansion between the parent metal andthe filler and, at least in a support zone thereof immediately adjacentthe positive pattern, are intrinsically self-bonding but only at atemperature above the melting point of the parent metal but below andsufficiently close to the oxidation reaction temperature to allow theaforesaid accommodation. Such self-bonding of the filler endows it withsufficient cohesive strength to retain the inversely replicated negativepattern against pressure differentials which develop across it bymovement of the parent metal into the filler.

Generally, as noted above, the filler may be a self-bonding filler inany case, though it need not necessarily be such in all cases.

It is not necessary that the entire mass or bed of filler comprise aconformable filler or, when required, a self-bonding filler, althoughsuch arrangement is within the purview of the invention. The filler needbe conformable and/or self-bondable only in that portion of the bed offiller adjacent to and shaped by the positive pattern of parent metal.In other words, the filler need be conformable and/or self-bondable onlyto a depth sufficient, in the case of conformability, to conform to thepositive pattern of the parent metal precursor, and, in the case ofself-bondability, to provide sufficient mechanical strength in aparticular situation. The balance of the filler bed need not beconformable and/or self-bonding.

In any case, the filler should not sinter, fuse or react in such a wayas to form an impermeable mass so as to block the infiltration of theoxidation reaction product therethrough or, when a vapor-phase oxidantis used, passage of such vapor-phase oxidant therethrough. Further, thefiller should be sufficiently conformable to accommodate the thermalexpansion differential between the parent metal and the filler uponheating of the assembly, and the volume change of the metal upon meltingthereof while retaining close conformity to the positive pattern of theparent metal precursor.

In practicing the process of this invention, the assembly of the parentmetal, bed of filler and, if used, barrier means or growth preventivemeans, is heated to a temperature above the melting point of the metalbut below the melting point of the oxidation reaction product, toprovide a body or pool of molten metal in an oxidizing environment. Oncontact with the oxident, the molten metal will react to form a layer ofoxidation reaction product. Upon continued exposure to the oxidizingenvironment, within an appropriate temperature region, the remainingmolten metal is progressively drawn into and through the oxidationreaction product in the direction of the oxidant and, on contact withthe oxidant, forms additional oxidation reaction product. At least aportion of the oxidation reaction product is maintained in contact withand between the molten parent metal and the oxidant so as to causecontinued growth of the polycrystalline oxidation reaction product inthe bed of filler, thereby embedding filler within the polycrystallineoxidation reaction product. The polycrystalline matrix materialcontinues to grow so long as suitable oxidation reaction conditions aremaintained.

The process is continued until the oxidation reaction product hasinfiltrated and embedded the desired amount of filler. The resultingceramic composite product includes filler embedded by a ceramic matrixcomprising a polycrystalline oxidation reaction product and including,optionally, one or more non-oxidized or metallic constituents of theparent metal, or voids, or both. Typically, in these polycrystallineceramic matrices, the oxidation reaction product crystallites areinterconnected in more than one dimension, preferably in threedimensions, and the metal inclusions or voids may be partiallyinterconnected. When the process is not conducted beyond the exhaustionof the parent metal, the ceramic composite obtained is dense andessentially void-free. When the process is taken to completion, that is,when as much of the metal as possible under the process conditions hasbeen oxidized, pores in place of the interconnected metal will haveformed in the ceramic composite. The resulting ceramic composite productof this invention possesses substantially the original dimensions and(the negative of) the geometric configuration of the positive patternsection of the parent metal precursor, adjusted for melting point andthermal expansion differential volume changes during processing of theparent metal precursor with respect to the composite body formed andcooled.

Referring now to the drawings, it should be noted that all elementsthereof are not necessarily to scale. For example, in FIGS. 9-11 thethickness of the illustrated paper or thin cardboard components isexaggerated for improved clarity of illustration. FIG. 1 shows a parentmetal precursor 2 shaped to have a pattern formed therein, referred toas a positive pattern, which essentially comprises a rectangular groove4 and a cylindrical shaped cavity 6, which can be a smooth bore asillustrated or a threaded bore, formed in a surface 8 and a rectangularland 9 projecting upwardly (as viewed in FIG. 1) from surface 8. Groove4, cavity 6 and land 9 are formed in surface 8 of the parent metalprecursor 2 and together therewith comprise the positive pattern whichwill be inversely replicated as described below in connection with theceramic article of FIG. 3. Parent metal precursor 2 also has a shoulderflange 11 extending from side 7a thereof, one side of shoulder flange 11being flush with and forming an extension of surface 8. The remainder ofparent metal precursor 2 comprises the surface 10 (FIG. 1A) which isopposite surface 8 and the four sides 7a, 7b, (FIG. 1) 7c and 7d (FIGS.1A and 2). Surface 10, sides 7a-7b and the portion of shoulder flange 11not comprising part of surface 8 comprise the non-replicating section ofparent metal precursor 2 when the interface between particulate inertmaterial 16 and filler 14 is at plane X--X (FIG. 2) as described below.As used herein and in the appended claims, the term "inert material"refers to a particulate material which is substantially inert to andnon-wettable by the molten parent metal under the process conditions,i.e., the melting and oxidation reaction conditions.

FIG. 2 shows parent metal precursor 2 placed within a refractory vessel12, such as an alumina vessel, containing a two-layer bed of particulatematerial, the lower portion of vessel 12 being filled with a conformablefiller 14 and the upper portion of vessel 12 (generally, above the planeX--X) being filled with a conformable, inert material 16. Thenon-replicating section of parent metal precursor 2 is that portionthereof which is overlaid by the inert material 16 and consequently isfree from contact with the bed of filler 14. Parent metal precursor 2may comprise any suitable parent metal, for example, aluminum parentmetal. Parent metal precursor 2 is positioned with its positive pattern4, 6, 8, 9 in conforming engagement with the bed 14 of conformablefiller, so that the conformable filler fills groove 4 and cylindricalshaped cavity 6 and abuts surface 8 and the surfaces of land 9,conforming to the respective changes of the positive pattern. Theconformable filler 14 thus extends above the plane X--X only withingroove 4 and cylindrical cavity 6. The non-replicating section of parentmetal precursor 2 is thus embedded within the inert material 16. Theconformable filler 14 does not extend beyond the opposite open ends ofgroove 4 so that at the opposite ends of groove 4 there is an interfacebetween conformable filler 14 and inert material 16. If necessary ordesirable, a suitable retaining means such as a paper, cardboard,plastic film, metal plate (preferably a perforated metal plate) orscreen may be placed at each opposite end of groove 4 in order topreclude seepage of conformable filler 14 and/or intermingling of inertmaterial 16 with conformable filler 14 during assembly.

Upon heating of the assembly of FIG. 2 to a sufficiently hightemperature to melt the parent metal of precursor 2, a vapor-phaseoxidant, which permeates the bed of barrier material and conformablefiller and therefore is in contact with the molten metal, oxidizes themolten metal and growth of the oxidation reaction product resultingtherefrom infiltrates the bed of conformable filler 14. The growingoxidation reaction product will not penetrate inert material 16 whichtherefore serves effectively to retain the molten metal for growth ofoxidation reaction product therefrom. For example, when the parent metalis an aluminum parent metal and air is the oxidant, the oxidationreaction temperature may be from about 850° C. to about 1450° C.,preferably from about 900° C. to about 1350° C., and the oxidationreaction product is alumina, typically alpha-alumina. The molten metalmigrates through the forming skin of oxidation reaction product from thevolume formerly occupied by parent metal precursor 2 and, as thereaction continues, the space within inert material bed 16 formerlyoccupied by parent metal precursor 2 is partially or substantiallyentirely evacuated by the migration of molten parent metal through theoxidation reaction product to the outer surface thereof, where itcontacts the vapor-phase oxidant within the bed of conformable filler 14and is oxidized to form additional oxidation reaction product. Movementof particles of inert material 16 into the space evacuated by moltenparent metal, i.e., into the initial location of parent metal precursor2, is acceptable as it should have no adverse effect on the growingceramic body. However, if desired or necessary due to the geometry ofthe positive pattern utilized, a rigid retainer means may be used topreclude such movement. For example, a suitable rigid retainer could beplaced on surface 10 of parent metal precursor 2 to retain theparticulate inert material 16 in place as molten parent metalinfiltrates the bed of filler 14.

The resultant oxidation reaction product comprises a polycrystallineceramic material which may contain inclusions therein of unoxidizedconstituents of the molten parent metal. Upon completion of the desiredamount of growth of ceramic matrix, the assembly is allowed to cool andthe resultant ceramic composite, whose dimensions are indicated bydotted line 18 in FIG. 2, is separated from the inert material 16 andexcess conformable filler and unreacted parent metal, if any, leftwithin vessel 12. Unreacted parent metal, if any, and any thin layer ofoxide formed at the interface with inert material 16 can readily beseparated from the ceramic composite. The ceramic composite structurethus formed will inversely replicate the shape of the positive patternand the remainder of the ceramic body may be shaped as desired bymachining or grinding or otherwise forming it to a desired outer shape.For example, as illustrated in FIG. 3, the finish-shaped ceramiccomposite body 20 has a replicated surface, i.e., a negative pattern,which is the negative of the positive pattern defined by groove 4,cavity 6, surface 8 and flange 9 of parent metal precursor 2. Thereplicated negative pattern of ceramic composite body 20 includes a slot21, which is the replicated negative pattern of flange 9, and acylindrical shaped boss 22 which is the replicated negative pattern ofcavity 6. The dimensions of slot 21 are congruent to those of flange 9and the dimensions of boss 22 are congruent to those of cavity 6.Similarly, a rectangular-shaped land 24 is congruent to, and comprisesthe inversely replicated negative pattern of, groove 4. Surface 26 ofcomposite body 20 is likewise the inversely replicated negative patternof surface 8 of parent metal precursor 2. The remaining portions ofcomposite body 20, e.g., sides 28a and 28b, plus the two sides (notvisible in FIG. 3) respectively opposite sides 28a and 28b, and thesurface (not visible in FIG. 3) opposite surface 26 are formed bymachining, grinding or otherwise shaping the generally loaf-shapedexterior portion of the ceramic body grown below plane X--X, whose shapeis generally indicated in FIG. 2 by dash line 18. Because shoulderflange 11 is embedded within inert material 16 (when the interfacebetween the inert material 16 and filler 14 is at plane X--X), with onlythe portion of shoulder flange 11 which comprises an extension ofsurface 8 in contact with filler 14, shoulder flange 11 is notreplicated in ceramic body 20. The effect of shoulder flange 11 in thisembodiment is to increase the length of ceramic body 20 (as measuredalong its major longitudinal axis) because the area of conformableengagement of filler 14 with precursor 2 (at surface 8) is increased bythe width of shoulder flange 11. For example, ignoring anyforeshortening occasioned by grinding the ceramic body 20 to provide thefinished surfaces 28, 28a, etc. thereof, the length of ceramic body 20between land 24 and side surface 28a thereof is shown in FIG. 3 by thedimension L' which is substantially the same as dimension L in FIG. 1.If shoulder flange 11 were omitted from parent metal precursor 2, lengthL' of ceramic body 20 (FIG. 3) would be substantially the same asdimension s in FIG. 1.

By selecting an appropriate material for the filler and maintaining theoxidation reaction conditions for a time sufficient to evacuatesubstantially all the molten parent metal from the barrier meanscomprised of bed 16 in the illustrated embodiment, a faithful inversereplication of the positive pattern of parent metal presursor 2 isattained by surface 26, land 24, boss 22 and slot 21 of ceramic body 20.If a quantity of unreacted parent metal remains on the ceramic body, itcan readily be removed from the resultant ceramic body to expose thefaithful inverse replication. While the illustrated shape of the parentmetal precursor 2 (and therefore of the replicated shape 21, 22, 26, 24)is relatively simple, positive patterns of much more complex geometrycan be formed in parent metal precursor 2 and inversely replicated withfidelity as the negative pattern of the composite ceramic body by thetechniques of the present invention.

In an alternate embodiment, the parent metal precursor 2 could beembedded more deeply within the bed of conformable filler 14, or theheight of bed 14 increased, to the level indicated by plane Y--Y, or toany level intermediate planes X--X and Y--Y. Conformable filler 14 couldeven extend above the level of plane Y--Y and cover a portion of surface10 of parent metal precursor 2 provided that a portion thereof is leftfree from contact with the filler to avoid formation of a cavity totallyenclosed by oxidation reaction product. The size of the positive patternsection increases as the height of the bed 14 of filler increases, toinclude that portion of the sides 7a, 7b, 7c and 7d of parent metalprecursor 2 which is embedded by conformable filler 14. Growth ofoxidation reaction product would then occur not only through surface 8and the surfaces of groove 4, cavity 6 and flange 9, but also throughthat portion of the sides 7a-7d of parent metal precursor 2 surroundedby and in contact with filler 14. In such case, the non-replicatingsection of parent metal precursor 2 would be that portion left clear offiller 14 such as, for example, only surface 10 of parent metalprecursor 2 when conformable filler 14 extends to plane Y--Y.

FIG. 4 shows in sectional perspective view a ceramic body 30 resultingfrom practicing the invention with the assembly of FIG. 4 in which theinterface between filler 14 and inert material 16 is at plane Y--Y sothat filler 14 is in conforming engagement with every surface of metalprecursor 2 except surface 10. In this arrangement, surface 10 comprisesthe entirety of the non-replicating section of parent metal precursor 2whose positive pattern is comprised of surface 8 and sides 7a, 7b, 7cand 7d and thus includes, in addition to groove 4, cavity 6 and flange9, shoulder flange 11. Carrying out the process of the invention withfiller 14 extending to the level of plane Y--Y results in growth ofoxidation reaction product to form a ceramic composite body generally asshown by dash line 19 in FIG. 2. The resultant cermic body 30, afterbeing separated from excess filler 14 and inert material 16, is shown inFIG. 4 before being ground or machined (if desired) along surfacesgenerally analogous to the side surfaces 28a, 28b and adjacent side andbottom surfaces (not visible in FIG. 3) of ceramic body 20 of FIG. 3.Ceramic body 30 is shown in FIG. 4 in the condition in which it isremoved from vessel 12, and has outer side surface 32, a bottom surface34 (as viewed in FIG. 4) and interior wall surfaces 36a, 36b and 36cwhich, respectively, comprise negative patterns inversely replicatingside surfaces 7a, 7b and 7c of parent metal precursor 2. (The interiorwall surface inversely replicating side surface 7d of parent metalprecursor 2 is omitted from the section view of FIG. 4, which is takenalong a plane parallel to but inwardly of the omitted interior wallinversely replicating side surface 7d.) Growth of the oxidation reactionproduct through those portions of filler 14 in conforming engagementwith side surfaces 7a-7d in FIG. 2 results in the formation of facinginterior walls 36a, 36b, 36c, and a fourth interior wall (not shown,which inversely replicates surface 7d) to provide a generallyrectangular-shaped recess 38 defined by the aforesaid interior walls andsurface 26'. Surface 26' comprises a negative pattern inverselyreplicating surface 8 of precursor 2 and generally corresponds tosurface 26 of the FIG. 3 embodiment. Surface 26' has therein a slot 21',boss 22' and land 24' generally corresponding to slot 21, boss 22 andland 24 of the FIG. 3 embodiment. In addition, ceramic body 30 has, atthe foot of interior wall 36, a slot or channel 40 which is the negativepattern inversely replicating shoulder flange 11 of precursor 2. Ceramicbody 30 optionally may be finished by, e.g., being ground or machined toprovide flat surfaces as generally suggested by the dash lines(unnumbered) in FIG. 4.

It will be appreciated upon consideration of the foregoing descriptionof the different shaped ceramic bodies obtained by changing the relativeposition of precursor 2 to the interface between filler 14 and inertmaterial 16, that the molten parent metal provided by precursor 2 willmigrate and grow as the oxidation reaction product into the bed offiller 14 through those areas of precursor 2 which are in contact withor engage a surface of precursor 2. Assuming the presence of materialsand conditions to provide growth of oxidation reaction product throughall surfaces of precursor 2 which are not blocked by contact with abarrier means, it will be appreciated that molten parent metal willevacuate the volume originally occupied by precursor 2 and grow asoxidation reaction product into filler 14, faithfully inverselyreplicating in the resultant self-supporting ceramic composite body theconfiguration of the interface between the positive pattern of parentmetal precursor 2 and the permeable filler 14 placed in conformingengagement therewith. For example, if the interface between filler 14and inert material 16 were placed at a level between planes X--X andY--Y, the height of interior walls 36a, 36b, 36c and the interior wallreplicating surface 7d, and thus the depth of recess 38, would bereduced correspondingly. For example, if the interface between filler 14and inert material 16 were at plane Z--Z, the height of the aforesaidinterior walls would be less than that of boss 22' or land 24'.

It should be understood that the filler properties of being permeableand conformable as described above are properties of the overallcomposition of the filler and that individual components of the fillerneed not have any or all of these characteristics. Thus, the filler maycomprise either a single material, a mixture of particles of the samematerial but of different mesh size, or mixtures of two or morematerials. In the latter case, some components of the filler may not besufficiently conformable or permeable but the filler of which it is acomponent part will have the requisite conformity or permabilitycharacteristics because of the presence of other materials. A largenumber of materials which make useful fillers in the ceramic compositeby imparting desired qualities to the composite also will have thepermeable and conformable qualities described above.

With respect to individual components of the filler, one suitable classof filler component includes those chemical species which, under thetemperature and oxidizing conditions of the process, are not volatile,are thermodynamically stable and do not react with or dissolveexcessively in the molten parent metal. Numerous materials are known tothose skilled to the art as meeting such criteria in the case where analuminum parent metal is employed with air or oxygen as the oxidant.Such materials include the single-metal oxides of: aluminum, Al₂ O₃ ;cerium, CeO₂ ; hafnium, HfO₂ ; lanthanum, La₂ O₃ ; neodymium, Nd₂ O₃ ;praseodymium, various oxides; samarium, Sm₂ O₃ ; scandium, Sc₂ O₃ ;thorium, ThO₂ ; uranium, UO₂ ; yttrium, Y₂ O₃ ; and zirconium, ZrO₂. Inaddition, a large number of binary, ternary, and higher order metalliccompounds such as magnesium aluminate spinel, MgO.Al₂ O₃, are containedin this class of stable refractory compounds.

A second class of suitable filler components are those which are notintrinsically stable in the oxidizing and high temperature environmentof the preferred embodiment, but which, due to relatively slow kineticsof the degradation reactions, can be incorporated as a filler phasewithin the growing ceramic body. An example is silicon carbide. Thismaterial would oxidize completely under the conditions necessary tooxidize, for example, aluminum with oxygen or air in accordance with theinvention were it not for a protective layer of silicon oxide formingand covering the silicon carbide particles to limit further oxidation ofthe silicon carbide. The protective silicon oxide layer also enablessilicon carbide particles to sinter or bond lightly to themselves and toother components of the filler under the oxidation reaction conditionsof the process for aluminum parent metal with air or oxygen as theoxidant.

A third class of suitable filler components are those, such as carbonfibers, which are not, on thermodynamic or on kinetic grounds, expectedto survive the oxidizing environment or the exposure to molten aluminuminvolved with a preferred embodiment, but which can be made compatiblewith the process if 1) the environment is made less active, for example,through the use of CO/CO₂ as the oxidizing gases, or 2) through theapplication of a coating thereto, such as aluminum oxide, which makesthe species kinetically non-reactive in the oxidizing environment or onexposure to the molten metal.

As a further embodiment of the invention and as explained in theCommonly Owned Patent Applications, the addition of dopant materials tothe metal can favorably influence the oxidation reaction process. Thefunction or functions of the dopant material can depend upon a number offactors other than the dopant material itself. These factors include,for example, the particular parent metal, the end product desired, theparticular combination of dopants when two or more dopants are used, theuse of an externally applied dopant in combination with an alloyeddopant, the concentration of the dopant, the oxidizing environment, andthe process conditions.

The dopant or dopants (1) may be provided as alloying constituents ofthe parent metal, (2) may be applied to at least a portion of thesurface of the parent metal, or (3) may be applied to the filler or to apart of the filler bed, e.g., to the depth of filler necessary toconform to the positive pattern of the parent metal precursor, or anycombination of two or more of techniques (1), (2) and (3) may beemployed. For example, an alloyed dopant may be used in combination withan externally applied dopant. In the case of technique (3), where adopant or dopants are applied to the filler, the application may beaccomplished in any suitable manner, such as by dispersing the dopantsthroughout part or the entire mass of filler as coatings or inparticulate form, preferably including at least a portion of the bed offiller adjacent the parent metal. Application of any of the dopants tothe filler may also be accomplished by applying a layer of one or moredopant materials to and within the bed, including any of its internalopenings, interstices, passageways, intervening spaces, or the like,that render it permeable. A convenient manner of applying any of thedopant material is to merely soak the entire bed in a liquid (e.g., asolution), of dopant material. A source of the dopant may also beprovided by placing a rigid body of dopant in contact with and betweenat least a portion of the parent metal surface and the filler bed. Forexample, a thin sheet of silicon-containing glass (useful as a dopantfor the oxidation of an aluminum parent metal) can be placed upon asurface of the parent metal. When the aluminum parent metal (which maybe internally doped with Mg) overlaid with the silicon-containingmaterial is heated in an oxidizing environment (e.g., in the case ofaluminum in air, between about 850° C. to about 1450° C., preferablyabout 900° C. to about 1350° C.), growth of the polycrystalline ceramicmaterial into the permeable bed occurs. In the case where the dopant isexternally applied to at least a portion of the surface of the parentmetal, the polycrystalline oxide structure generally grows within thepermeable filler substantially beyond the dopant layer (i.e., to beyondthe depth of the applied dopant layer). In any case, one or more of thedopants may be externally applied to the parent metal surface and/or tothe permeable bed. Additionally, dopants alloyed within the parent metaland/or externally applied to the parent metal may be augmented bydopant(s) applied to the filler bed. Thus, any concentrationdeficiencies of the dopants alloyed within the parent metal and/orexternally applied to the parent metal may be augmented by additionalconcentration of the respective dopant(s) applied to the bed, and viceversa.

Useful dopants for an aluminum parent metal, particularly with air asthe oxidant, include, for example, magnesium metal and zinc metal, incombination with each other or in combination with other dopants asdescribed below. These metals, or a suitable source of the metals, maybe alloyed into the aluminum-based parent metal at concentrations foreach of between about 0.1-10% by weight based on the total weight of theresulting doped metal. Concentrations within this range appear toinitiate the ceramic growth, enhance metal transport and favorablyinfluence the growth morphology of the resulting oxidation reactionproduct. The concentration for any one dopant will depend on suchfactors as the combination of dopants and the process temperature.

Other dopants which are effective in promoting polycrystalline oxidationreaction growth, for aluminum-based parent metal systems are, forexample, silicon, germanium, tin and lead, especially when used incombination with magnesium or zinc. One or more of these other dopants,or a suitable source of them, is alloyed into the aluminum parent metalsystem at concentrations for each of from about 0.5 to about 15% byweight of the total alloy; however, more desirable growth kinetics andgrowth morphology are obtained with dopant concentrations in the rangeof from about 1-10% by weight of the total parent metal alloy. Lead as adopant is generally alloyed into the aluminum-based parent metal at atemperature of at least 1000° C. so as to make allowances for its lowsolubility in aluminum; however, the addition of other alloyingcomponents, such as tin, will generally increase the solubility of leadand allow the alloying material to be added at a lower temperature.

One or more dopants may be used depending upon the circumstances, asexplained above. For example, in the case of an aluminum parent metaland with air as the oxidant, particularly useful combinations of dopantsincludes (a) magnesium and silicon or (b) magnesium, zinc and silicon.In such examples, a preferred magnesium concentration falls within therange of from about 0.1 to about 3% by weight, for zinc in the range offrom about 1 to about 6% by weight, and for silicon in the range of fromabout 1 to about 10% by weight.

Where the parent metal is aluminum internally doped with magnesium andthe oxidizing medium is air or oxygen, it has been observed thatmagnesium is at least partially oxidized out of the alloy attemperatures of from about 820° to 950° C. In such instances ofmagnesium-doped systems, the magnesium forms a magnesium oxide and/ormagnesium aluminate spinel phase at the surface of the molten aluminumalloy and during the growth process such magnesium compounds remainprimarily at the initial oxide surface of the parent metal alloy (i.e.,the "initiation surface") in the growing ceramic structure. Thus, insuch magnesium-doped systems, an aluminum oxide-based structure isproduced apart from the relatively thin layer of magnesium aluminatespinel at the initiation surface. Where desired, this initiation surfacecan be readily removed as by grinding, machining, polishing or gritblasting. In addition, an extremely thin (e.g., less than about 2 μm)layer of magnesium oxide has been observed on the external surface whichcan be easily removed as by grit blasting, if desired.

Additional examples of dopant materials useful with an aluminum parentmetal, include sodium, lithium, calcium, boron, phosphorus and yttriumwhich may be used individually or in combination with one or moredopants depending on the oxidant and process conditions. Sodium andlithium may be used in very small amounts in the parts per millionrange, typically about 100-200 parts per million, and each may be usedalone or together, or in combination with other dopant(s). Rare earthelements such as cerium, lanthanum, praseodymium, neodymium and samariumare also useful dopants, and herein again especially when used incombination with other dopants.

As noted above, it is not necessary to alloy any dopant material intothe parent metal. For example, selectively applying one or more dopantmaterials in a thin layer to either all, or a portion of, the surface ofthe parent metal enables local ceramic growth from the parent metalsurface or portions thereof and lends itself to growth of thepolycrystalline ceramic material into the permeable filler in selectedareas. Thus, growth of the polycrystalline ceramic material into thepermeable bed can be controlled by the localized placement of the dopantmaterial upon the parent metal surface. The applied coating or layer ofdopant is thin relative to the thickness of the parent metal body, andgrowth or formation of the oxidation reaction product into the permeablebed extends to substantially beyond the dopant layer, i.e., to beyondthe depth of the applied dopant layer. Such layer of dopant material maybe applied by painting, dipping, silk screening, evaporating, orotherwise applying the dopant material in liquid or paste form, or bysputtering, or by simply depositing a layer of a solid particulatedopant or a solid thin sheet or film of dopant onto the surface of theparent metal. The dopant material may, but need not, include eitherorganic or inorganic binders, vehicles, solvents, and/or thickeners.More preferably, the dopant materials are applied as powders to thesurface of the parent metal or dispersed through at least a portion ofthe filler. One particularly preferred method of applying the dopants tothe parent metal surface is to utilize a liquid suspension of thedopants in a water/organic binder mixture sprayed onto a parent metalsurface in order to obtain an adherent coating which facilitateshandling of the doped parent metal prior to processing.

The dopant materials when used externally are usually applied to aportion of a surface of the parent metal as a uniform coating thereon.The quantity of dopant is effective over a wide range relative to theamount of parent metal to which it is applied and, in the case ofaluminum, experiments have failed to identify either upper or loweroperable limits. For example, when utilizing silicon in the form ofsilicon dioxide externally applied as the dopant for an aluminum-basedparent metal when using air or oxygen as the oxidant, quantities as lowas about 0.0001 gram of silicon per square centimeter of externallydoped surface of parent metal or about 0.00003 gram of silicon per gramof parent metal to be oxidized may be employed to produce thepolycrystalline ceramic growth phenomenon. One or more other dopants maybe used, for example, the silicon dopant material may be supplemented bya dopant material comprising a source of magnesium and/or zinc. It alsohas been found that a ceramic structure is achievable from analuminum-based parent metal using air or oxygen as the oxidant by usingone or both of MgO and MgAl₂ O₄ as the dopant in an amount greater thanabout 0.003 gram of Mg per square centimeter of externally doped surfaceof parent metal or greater than about 0.0008 gram of Mg per gram ofparent metal to be oxidized.

Dopant application techniques (2) and (3) described above, i.e.,external application of dopant to at least a portion of the surface ofparent metal or to the filler bed or part of the filler bed, may beutilized in an embodiment of the invention in which growth control ofthe oxidation reaction product is attained by such external applicationof the dopant. Materials and conditions may be selected such thatsignificant growth of oxidation reaction product will not occur fromthose portions of the parent metal precursor lacking the external dopantand the parent metal precursor is not alloyed with sufficient dopant tofacilitate the oxidation reaction. When an external dopant is used inconjunction with the positive pattern section only of the parent metalprecursor, the barrier means may be omitted from the non-replicatingsection. However, it is to be understood that the external applicationof the dopant may also be used in combination with a barrier material.

The technique of utilizing an external dopant is illustrated in FIG. 5,wherein parent metal precursor 2 is embedded within a bed of conformablefiller 14, with all surfaces of parent metal precursor 2, including thenon-replicating section thereof, in conforming engagement withconformable filler 14. This type of embedment would be attained, forexample, by replacing the bed of inert material 16 in FIG. 2 withconformable filler so that the refractory vessel 12 is entirely filledwith a bed of conformable filler 14 having parent metal precursor 2embedded therein. In the FIG. 5 embodiment, external application of adopant is utilized to give the same effect as would be attained in theFIG. 2 embodiment if the interface between the bed 14 of conformablefiller material and the bed 16 of particulate inert material were at thelevel of plane X--X. In order to attain this effect, a layer 40 ofdopant material is applied to cover the entire surface of the positivepattern section comprised of surface 8 which, as described above withrespect to the embodiments of FIGS. 1-4, has a groove 4, cavity 6 andflange 9 formed therein, with shoulder flange 11 forming an extensionthereof. Surfaces 10, 7a, 7c, 7b and 7d and the surfaces of shoulderflange not coated with dopant material 40 together comprise thenon-replicating section of parent metal precursor 2 in the embodimentillustrated in FIG. 5 (surface 7b is not visible in FIG. 5). Theoxidation reaction conditions utilized with the FIG. 5 embodiment aresuch that layer 40 of dopant material is required to promote growth ofthe oxidation reaction product and, in the absence of the layer 40 ofdopant material, growth of oxidation reaction product is precluded orinhibited sufficiently to avoid any significant formation of oxidationreaction product from the surfaces of parent metal precursor 2comprising the non-replicating section thereof. Thus, in thisembodiment, parent metal precursor 2 would contain no or insufficientalloyed dopant to promote growth of oxidation reaction product under theconditions obtaining. Factors such as the composition of the parentmetal, the composition and amount of oxidant, and the operatingtemperature will determine whether a particular parent metal requiresthe presence of a dopant in order to form oxidation reaction product atan appreciable rate. With the arrangement shown in FIG. 5, and underconditions wherein the layer 40 of dopant material is required topromote significant oxidation reaction product growth, no significantgrowth will occur from the non-replicating section even though it is inconforming engagement with a bed of conformable filler 14 which ispermeable to growth of oxidation reaction product therethrough. In lieuof, or in addition to the layer 40 of dopant material, a suitable dopantmay be utilized in those portions or zones of the bed 14 of conformablefiller facing, adjacent to and/or contiguous with the positive patternsection of parent metal precursor 2. Still further, a solid or liquidoxidant may be used in such zones of the bed of filler to establishfavorable growth kinetics at the positive pattern section. The productresulting from the assembly partially illustrated in FIG. 5 would besimilar or identical to the ceramic composite body illustrated in FIG.3.

FIG. 6 shows another embodiment of the invention in which a parent metalprecursor 2' is embedded within a bed 14 of conformable filler whichitself is retained within a generally rectangular enclosure 42 made of amaterial comprising a foraminous barrier material. Enclosure 42 issubstantially filled with conformable filler 14 and parent metalprecursor 2' embedded therein. The foraminous barrier material of whichenclosure 42 is made may comprise, for example, a stainless steelscreen. The enclosure 42 has a circular opening formed in its upper andlower surfaces 42a, 42b (see FIG. 6) and a pair of circular cylindricalshaped tubes 44a, 44b are inserted through these openings and extend torespective opposite surfaces 46, 48 of parent metal precursor 2'. Tubes44a, 44b are each filled with an inert material 16, and the tubes maythemselves be formed of a foraminous barrier material or a screenidentical or similar to that of enclosure 42. Parent metal precursor 2'has in this embodiment a flange 50 protruding from surface 48 thereof.Visible in FIG. 6 are side surfaces 52a , 52c and front surface 52d ofparent metal precursor 2'. ("Side" and "front" are used in the foregoingsentence as viewed in FIG. 6). The back (as viewed in FIG. 6) surface ofparent metal 2' is not visible in FIG. 6. It is to be understood thatall described surfaces of parent metal precursor 2' are in conformingengagement with filler 14 contained within the enclosure 42 except forcircular portions of opposite surfaces 46 and 48 which are overlaid bythe particles of inert material 16 contained within, respectively, tubes44a and 44b. Thus, the entire surface of parent metal precursor 2'comprises the positive section thereof except for the two circularsegments overlaid by inert material 16, which segments compriserespective non-replicating sections of parent metal precursor 2'.Inasmuch as enclosure 42 provides a barrier to growth of oxidationreaction product, the bed 15 of particulate material need be neither aconformable filler nor an inert material. Indeed, the assembly comprisedof the enclosure 42 and tubes 44a, 44b may be supported by any suitablemeans within refractory vessel 42. It is convenient, however, to supportthe assembly in a bed of particulate material 15 which may, but need notbe, an inert material. If the enclosure 42 were not itself a barrier togrowth of oxidation reaction product then bed 15, or at least theportion thereof adjacent to and embedding enclosure 42, should comprisean inert material.

Upon heating of the assembly of FIG. 6 to a sufficiently hightemperature to melt the parent metal, and upon contact of the moltenparent metal with a suitable liquid, solid and/or vapor-phase oxidant,oxidation of the molten metal take place and growth of oxidationreaction product from the positive pattern section of parent metalprecursor 2' takes place. As the reaction is allowed to progress toattain the desired growth of the ceramic body (optionally, to theexhaustion of parent metal from the volume initially occupied by parentmetal precursor 2'), oxidation reaction product will grow to a boundarydefined by the inner surface of the enclosure 42. The volume ofenclosure 42 relative to the volume of parent metal precursor 2' isreadily selected so that a volume of oxidation reaction product willresult which will fill the interstices of the volume of conformablefiller 14 contained within the enclosure 42.

FIG. 7 shows a resultant ceramic composite body 54 obtained by utilizingthe assembly of FIG. 6. Ceramic composite body 54 has a generally flat,top surface 56 and side surfaces 58, 60, visible in FIG. 7. Thesesurfaces generally conform to the corresponding interior surfaces of theenclosure 42. A cylindrical opening 62a extends to top surface 56 andgenerally corresponds to the volume of tube 44a contained withinenclosure 42. A corresponding cylindrical opening 62b extends to thebottom surface (unnumbered) of ceramic composite body 54 and correspondsto the volume of tube 44b enclosed within enclosure 42. The volumeinitially occupied by parent metal precursor 2' is evacuated duringoxidation of the parent metal and results in a generally rectangularshaped cavity 64 formed within ceramic composite 54 and shown in dashoutline in FIG. 7. The lower surface (as viewed in FIG. 7) of cavity 64contains a groove 66 formed therein which is an inverse replication ofthe surface of flange 50 of parent metal precursor 2'. The tubes 44a,44b are filled with particles of an inert material 16 in the assembly ofFIG. 6. Since the inert material is permeable, it provides, via tubes44a, 44b, access to the surrounding atmosphere by the cavity 64 beingformed during the reaction, so that cavity 64 is at no time entirelyclosed and sealed off from the surrounding atmosphere by growingoxidation reaction product. As explained above, this avoids the problemof a pressure differential acting on the growing, hollow body ofoxidation reaction product due to the fact that the oxidation reactionproduct is impermeable to the surrounding air or atmosphere.

Referring now to FIGS. 8, 8A and 8B, there is shown another embodimentof a parent metal precursor 68, for example, an aluminum parent metalprecursor, which is of generally rectangular configuration, havingsurfaces 70, 74 and sides 72a, 72b, 72c and 72d. Parent metal precursor68 has a rectangular shaped land 76 which projects from its surface 74.Land 76 extends substantially parallel to and coextensively with sides72a and 72c. A cylindrically shaped bore 78 extends through parent metalprecursor 68, from surface 70 to surface 74 thereof.

FIG. 9 shows parent metal precursor 68 placed within a refractory vessel80 in an assembly of the parent metal precursor 68 with conformablefiller and a barrier means or growth preventive means. In thisembodiment, a cylindrical barrier means 82, which inhibits or preventsgrowth, is dimensioned and configured so that it may be slidablyinserted into cylindrical shaped bore 78 in engagement with the entirecylindrical surface thereof. As shown in FIGS. 8B and 9, cylindricalbarrier means 82 is longer than bore 78 and a portion thereof projectsoutwardly at either end thereof. The cross section view of FIG. 9 showsthe construction of cylindrical barrier means 82 which comprises, in theillustrated embodiment, a central core 82b, which may be made of plasterof paris, contained within a heavy paper or thin cardboard tube 82a usedto establish the initial configuration of the barrier. On heating, thepaper or cardboard burns off or volatilizes and does not participatefurther in the process. A rectangular shaped barrier means 88, open atits upper and lower ends (as seen in FIG. 9), is shown in cross sectionin FIG. 9 and is comprised of four walls which extend, respectively,parallel to and spaced from sides 72a, 72b, 72c and 72d of parent metalprecursor 68. Barrier means 88 thus has the shape of a short section ofa rectangular duct. Only three of the walls of means 88 are visible inFIG. 9, to wit, wall 88b and, in cross section, walls 88a and 88c. Asshown with respect to the latter two walls, the inner surface of each iscomprised of a layer of plaster of paris which, in walls 88a and 88c, isshown in cross section as layers 88a' and 88c'. The outer, heavy paperor cardboard layer is shown in cross section as layers 88a" and 88c".

Parent metal precursor 68 together with cylindrical barrier means 82inserted in the cylindrical bore thereof, is embedded within a bed ofconformable filler 84 contained within rectangular barrier means 88.Barrier means 88 and its contents are embedded within a bed of inertmaterial 86, from which it is separated by barrier means 88. In thisembodiment, the non-replicating section of parent metal precursor 68 isprovided by the cylindrical surface of cylindrical shaped bore 78, whichsurface engages and is congruent to the outer surface of cylindricallyshaped barrier means 82. The remaining surfaces of parent metalprecursor 68 comprise its positive pattern, as growth of oxidationreaction product from parent metal precursor 68 will, under suitableconditions as described above, occur from these surfaces through the bedof conformable filler 84. The growth of oxidation reaction product isconstrained to stop as the growing oxidation reaction product contactsbarrier means 82 and 88 and inert material 86, respectively. Thearrangement shown in FIG. 9 will produce a ceramic body having aconfiguration identical or substantially similar to that described aboveand illustrated in FIG. 7 as being obtained from the assembly of FIG. 6.Accordingly, it is not necessary to repeat the description of theceramic body of FIG. 7.

Referring now to FIGS. 10 and 11, there is shown another method ofobtaining a ceramic composite body similar or identical to thatillustrated in FIG. 3, which by use of suitable barrier means controlsthe extent of growth of the oxidation reaction provided and thus avoidsthe necessity of machining or grinding to the extent required to shapethe irregular portions of the ceramic body of FIG. 4 (which was formedby utilizing the assembly of FIG. 2). As shown in FIG. 10, a parentmetal precursor 2' is similar or identical in shape to the parent metalprecursor 2 of FIGS. 1, 1A and 2. Thus, parent metal precursor 2' has aflat surface 10', an opposite surface 8' from which extends arectangular land 9' and in which are formed a groove 4' and acylindrically shaped bore or cavity 6'. A shoulder flange 11' extendsalong one side of parent metal precursor 2' which is encased within arectangular barrier means 90 which comprises, in effect, a rectangularshaped heavy paper or thin cardboard box, open at its opposite ends.Rectangular barrier means 90 is lined with plaster of paris in a mannersimilar to that of rectangular barrier means 88 of the FIG. 9embodiment. Thus, as illustrated in FIG. 10, rectangular barrier means90 comprises walls 90a, 90b, 90c and 90d, most of wall 90d being brokenaway in FIG. 10 for improved clarity of illustration. Each of walls90a-90d has an interior lining of hardened plaster of paris as bestillustrated with respect to cross sectioned wall 90c which showscarboard outer wall 90c' having an inner lining of plaster of paris 90c"thereon. Similarly, as shown in FIG. 11, wall 90a is comprised ofcardboard 90a' having thereon a plaster of paris layer 90a". Surface 10'of parent metal precursor 2' has a coating 92 of plaster of parisapplied thereto.

Five of the six major surfaces of parent metal precursor 2' are thuscovered by a barrier means comprising, in the illustrated embodiment, alayer of plaster of paris. As with all the plaster of paris/cardboardbarrier means illustrated, the cardboard or paper serves as a form onwhich the plaster of paris may be applied in its wet or plastic state,and then allowed to dry to harden into a rigid barrier means. Thecardboard also serves to reinforce the plaster of paris barrier means tohelp prevent cracking or breakage during handling and assembling thebarrier means and parent metal precursor into the refractory vessel. Asindicated earlier, any other suitable materials may be substituted forthe paper or cardboard, and for the plaster of paris.

With growth of the oxidation reaction product thus inhibited orprecluded by the barrier means, surface 8', groove 4', bore 6' and land9' together comprise the positive pattern of parent metal precursor 2',the remaining surfaces thereof comprising the non-replicating section ofparent metal precursor 2'.

FIG. 11 shows parent metal precursor 2' and its associated barrier means90 embedded within a bed of particulate inert material 94 andcontaining, in the "freeboard" space above precursor 2', a conformablefiller 96. The upper portion (as viewed in FIGS. 10 and 11) ofrectangular barrier means 90 extends above the surface 8' of parentmetal precursor 2' and thus serves to separate the bed of conformablefiller 96 from the bed of particles of inert material 94 containedwithin refractory vessel 98. By heating the assembly of FIG. 11 to asuitable elevated temperature and maintaining it at that temperature fora sufficient period of time in accordance with the methods describedabove, a ceramic composite body similar or identical to that illustratedin FIG. 3 is obtained, as will be shown by the example given below.

The ceramic composite structures obtained by the practice of the presentinvention will usually be a dense, coherent mass wherein between about5% and about 98% by volume of the total volume of the compositestructure is comprised of one or more of the filler components embeddedwithin a polycrystalline ceramic matrix. The polycrystalline ceramicmatrix is usually comprised of, when the parent metal is aluminum andair or oxygen is the oxidant, about 60% to about 99% by weight (of theweight of polycrystalline matrix) of interconnected alpha-alumina andabout 1% to 40% by weight (same basis) of non-oxidized metallicconstituents, such as from the parent metal.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE 1

A parent metal precursor was machined to have the shape shown in FIGS.1, 1A and 10. The precursor was machined from a block of aluminum alloy380.1 obtained from Belmont Metals, Inc. and having a nominalcomposition of 8 to 8.5% by weight silicon, 2 to 3% by weight zinc, 0.1%by weight magnesium, 3.5% by weight copper as well as iron, manganeseand nickel, although the magnesium content was sometimes higher as inthe range of 0.17-0.18%. The resultant shaped parent metal precursor wasprovided with a barrier means as illustrated by barrier means 90, 92 inFIG. 10. The barrier means corresponding to 90 of FIG. 10 comprised acardboard form on which plaster of paris (Bondex, obtained from BondexCompany) was applied in a layer approximately 1/16 to 1/8 inches thick.The barrier means corresponding to 92 in FIG. 10 comprised a layer ofthe same plaster of paris, approximately 1/16 to 1/8 inches thick. Thus,the surfaces corresponding to surfaces 10, 7a, 7b, 7c and 7d of theparent metal precursor illustrated in FIGS. 1, 1A and 10 were coatedwith a barrier material and comprised the non-replicating section of theprecursor. Surface 8, groove 4, bore 6 and land 9 were free of thebarrier material and so comprised the positive pattern of the parentmetal precursor. The barrier means corresponding to 90 of FIG. 11extended approximately 5/8 of an inch above the surface 8 of the parentmetal precursor. A filler comprising a uniform admixture of aluminaparticles (38 Alundum obtained from Norton Company) comprising 70 weightpercent 220 grit particles and 30 weight percent 500 grit particles, andsilicon metal particles in the amount of 7% by weight of the totalweight of Alundum particles, was placed within the freeboard space abovethe precursor provided by the barrier means corresponding to 90 of FIG.10. The filler was thus placed in conforming engagement with thepositive pattern provided by the surface 8, groove 4, bore 6 and land 9.The assembly of the barrier means, filler and parent metal precursor wasplaced upon and embedded within a bed of inert material comprisingalumina particles (El Alundum obtained from Norton Company, of 90 meshsize) in the manner illustrated in FIG. 11. The bed of inert materialcorresponds to 94 of FIG. 11 and was substantially level with the top ofthe barrier enclosure means corresponding to 90 of FIG. 11.

The resulting assembly was placed into a furnace and heated in air at1000° C. for 28 hours. The assembly was allowed to cool and the ceramiccomposite body grown from the parent metal precursor was removed fromthe refractory vessel and excess filler and barrier material was removedtherefrom by light sandblasting. A ceramic body generally with the shapeillustrated in FIG. 3 was obtained, which showed high fidelity inversereplication of the positive pattern of the parent metal precursor.

EXAMPLE 2

A block of the same aluminum alloy as utilized in Example 1 was machinedand bored to provide a parent metal precursor having the shapeillustrated in FIGS. 8 and 8A and overall dimensions of 21/2 inches longby 11/4 inches wide by 11/16ths of an inch thick, with a cylindricalbore (corresponding to 78 of FIGS. 8 and 8A) being 3/4 inch in diameter.A rectangular land (corresponding to 76 of FIGS. 8 and 8A) measured1/16th inch thick (height above the surface corresponding to 74 of FIGS.8 and 8A) and 1/4 inch wide. A paper tube filled with plaster of paris(Bondex, from Bondex Company) was inserted into the bore with the outerdiameter of the paper tube congruent to and in contact with the surfaceof the cylindrical bore and the cylindrical barrier means extendingabout 1/4 inch out of each opposite end of the cylindrical bore. Plasterof paris (Bondex supplied by Bondex Company) was applied in a thicklayer to a heavy paper material in a shape of a rectangular box open atits opposite ends, the dimensions of the box being about 3 inches longby 11/2 inches wide and 11/4 inches high. This plaster of paris-coatedbox corresponds to barrier means 88 of FIG. 9.

A base layer of inert material comprising El Alundum, from NortonCompany, of 90 mesh size was placed within a refractory crucible. Oneopen end of the rectangular barrier means was placed upon the layer ofinert material, and the parent metal precursor (with the cylindricalbarrier means inserted in the bore thereof) was embedded within a bed offiller (corresponding to 84 of FIG. 9) contained within the rectangularbarrier means, substantially as shown in FIG. 9. The filler was the samefiller as used in Example 1 and substantially filled the rectangularbarrier means. The same type of inert material (corresponding to 86 ofFIG. 9) as used in Example 1 was added to approximately the same heightas the filler and the result was an assembly substantially asillustrated in FIG. 9. The resulting setup was placed into a furnace andheated in an air atmosphere at 1000° C. for 28 hours. After this period,the assembly was allowed to cool and the resulting ceramic compositebody obtained therefrom was removed from refractory vessel 80 and excessfiller and barrier material adhering to it were removed by lightsandblasting. The result was a ceramic body substantially as shown inFIG. 7 which faithfully inversely replicated the positive patternportion of the parent metal precursor.

In both Examples 1 and 2 the conformable filler placed in contact withthe positive pattern of the parent metal precursor is a self-bonding,conformable filler so that any pressure differential acting on theforming oxidation reaction product was resisted by the self-bondingnature of the filler. That is, if a pressure differential should occuracross the forming shell of oxidation reaction product because migrationof the molten parent metal to form additional oxidation reaction productleaves behind a cavity of reduced pressure, the self-bonding nature ofthe filler provides sufficient mechanical strength to resist themechanical forces imposed on the shell of forming oxidation reactionproduct by the pressure differential. However, in the two Examples, thethin layer of plaster of paris forming the barrier means wassufficiently permeable to air so that air permeated therethrough andequalized the pressure in the cavity or void formed by the migratingparent metal.

Although only a few exemplary embodiments of the invention have beendescribed in detail above, those skilled in the art will readilyappreciate that the present invention embraces many combinations andvariations other than those exemplified.

I claim:
 1. A self-supporting ceramic matrix composite body comprising aceramic matrix incorporating at least one filler material, said ceramicmatrix embedding said filler material and said ceramic matrix consistingessentially of about 60-99 percent by weight of an essentially singlephase polycrystalline oxidation reaction product consisting essentiallyof a material selected from the group consisting of silicon carbide,silicon boride, aluminum boride, titanium nitride, zirconium nitride,titanium boride, zirconium boride, tin oxide and aluminum oxynitride,and the remainder of said ceramic matrix consisting essentially of atleast one metallic constituent and voids.
 2. The ceramic matrixcomposite body of claim 1, wherein said metallic constituent comprisesat least one metal dispersed throughout said ceramic matrix, wherein atleast a portion of said metallic constituent is interconnected.
 3. Theceramic matrix composite body of claim 1, wherein said remainder of saidceramic matrix consists essentially of a metallic constituent and voids,said voids comprising at least 1% by volume of said ceramic matrix andsaid voids being dispersed throughout said ceramic matrix, wherein atleast a portion of said voids are interconnected.
 4. The ceramic matrixcomposite body of claim 1, wherein said filler material comprises atleast one material selected from the group consisting of aluminum oxide,silicon carbide, silicon aluminum oxynitride, zirconium oxide, zirconiumboride, titanium nitride, barium titanate, boron nitride, siliconnitride, zirconium nitride, titanium diboride and aluminum nitride.
 5. Aself-supporting ceramic matrix composite body consisting essentially offrom about 2% to about 95% by volume of a three-dimensionallyinterconnected ceramic matrix and from about 5% to about 98% by volumeof at least one filler incorporated within said matrix, said matrixconsisting essentially of from about 60% to about 99% by weight of athree-dimensionally interconnected oxidation reaction product consistingessentially of a material selected from the group consisting of siliconcarbide, silicon boride, aluminum boride, titanium nitride, zirconiumnitride, titanium boride, zirconium boride, tin oxide and aluminumoxynitride and the remainder of said matrix consisting essentially of atleast one interconnected metallic constituent and voids.
 6. The ceramicmatrix composite body of claim 5, wherein said oxidation reactionproduct consists essentially of silicon carbide and said metallicconstituent comprises silicon.
 7. The ceramic matrix composite body ofclaim 5, wherein said oxidation reaction product consists essentially ofsilicon boride and said metallic constituent comprises silicon.
 8. Theceramic matrix composite body of claim 5, wherein said oxidationreaction product consists essentially of aluminum boride and saidmetallic constituent comprises aluminum.
 9. The ceramic matrix compositebody of claim 5, wherein said oxidation reaction product consistsessentially of titanium nitride and said metallic constituent comprisestitanium.
 10. The ceramic matrix composite body of claim 5, wherein saidoxidation reaction product consists essentially of zirconium nitride andsaid metallic constituent comprises zirconium.
 11. The ceramic matrixcomposite body of claim 5, wherein said oxidation reaction productconsists essentially of titanium boride and said metallic constituentcomprises titanium.
 12. The ceramic matrix composite body of claim 5,wherein said oxidation reaction product consists essentially ofzirconium boride and said metallic constituent comprises zirconium. 13.The ceramic matrix composite body of claim 5, wherein said oxidationreaction product consists essentially of tin oxide and said metallicconstituent comprises tin.
 14. The ceramic matrix composite body ofclaim 5, wherein said oxidation reaction product consists essentially ofaluminum oxynitride and said metallic constituent comprises aluminum.15. The ceramic matrix composite body of claim 5, wherein said remainderof said ceramic matrix consists essentially of a metallic constituentand voids, said voids comprising at least 1% by volume of said ceramicmatrix and said voids being dispersed throughout said ceramic matrix,wherein at least a portion of said voids are interconnected.
 16. Theceramic matrix composite body of claim 5, wherein said filler materialcomprises at least one material selected from the group consisting ofaluminum oxide, silicon carbide, silicon aluminum oxynitride, zirconiumoxide, zirconium boride, titanium nitride, barium titanate, boronnitride, silicon nitride, zirconium nitride, titanium diboride andaluminum nitride.